The present invention relates to temperature regulation of an oven and particularly household ranges having a user positioned control selectively moved to a position indicative of a desired oven temperature. Typically, oven temperature regulators employ a liquid filled temperature sensing bulb and tube which utilizes the pressure forces of the expanding liquid in the bulb and tube to move a diaphragm disposed to actuate a switch or other control mechanism for controlling the oven heating unit or element. In ovens employing electric resistance heating units, the fluid actuated diaphragm typically operates a switch or relay for controlling current to the heating element. The user control is typically arranged to apply a preload or preset condition to the diaphragm/switch operating mechanism. In ovens employing a fuel gas burning heating unit or burner, the diaphragm may move to operate a mechanism for mechanically controlling fuel flow in a valve.
Alternatively, a bimetallic sensing element has been employed for sensing oven temperature for moving an electrical contact to open and close a circuit to an electric oven heating unit. In the case of a fuel gas burner oven heating unit, a bimetallic element has been employed to control a movable valve member for controlling fuel flow to the burner in response to the sensed oven temperature. In some applications the bimetallic element is heated by an electrical current responsive to a thermistor in the oven for sensing oven temperature; and the bimetallic element mechanically operates the fuel gas valve or an electrical relay for energizing an electrically operated fuel valve.
In the aforementioned control systems for regulating oven temperature, mechanical motion of either a heat pressure actuated diaphragm or a bimetallic member Provides the motion necessary to move a valve member or open and close a set of electrical contacts for controlling the oven heating element. The accuracy and repeatability of the aforementioned oven control thus depends on the mechanical sensitivity of the sensing element and the ability to calibrate such an element to provide the desired movement over a range of desired oven temperatures.
In ovens which provide a self-cleaning mode of operation the heating element is operated to raise the oven to a temperature substantially above that of normal service use for a period of time to oxidize carbonaceous food residue on the oven walls. In such a self-cleaning mode of operation, providing accurate calibration and accurate sensitivity over such a broadened range of temperatures has proven to be difficult in designing and manufacturing a low cost oven controller for high volume mass production of ovens. Problems have been encountered in sealing and calibrating the liquid filled tube or bulb; and, where highly corrosive and toxic liquid metal fills are employed, hazardous conditions result in manufacturing the liquid filled bulb.
Furthermore, it has been found that in the manufacture of bimetal sensors for oven thermostats a high degree of precision is required to control the composition of the bimetal element and the control of the thickness dimensions thereof in order to produce accurate and repeatable movement of the bimetal element over the range of temperatures experienced in the oven to provide repeatable mechanical motion at a given temperature.
It has thus been desired to find a way or means of inexpensively employing an electronic controller for providing response to changes in oven temperatures and to provide a control signal for energizing an electrical power actuator to control the oven heating unit in an effort to utilize the accuracy and reliability of electronic circuitry to eliminate the problems encountered in providing repeatability and the desired sensitivity of mechanical motion in an oven controller.